Light imaging, detection and ranging (LIDAR) systems measure distance to a target by illuminating the target with a pulsed laser light and measuring the reflected pulses with a sensor. Time-of-flight measurements can then be used to make a digital 3D-representation of the target. LIDAR systems can be used for a variety of applications where 3D depth images are useful including archaeology, geography, geology, forestry, mapping, construction, medical imaging and military applications, among others. Autonomous vehicles can also use LIDAR for obstacle detection and avoidance as well as vehicle navigation.
Many currently available LIDAR sensors that provide coverage and resolution sufficient for obstacle detection and avoidance in autonomous vehicles are both technologically complex and costly to manufacture. Such sensors are thus too expensive to allow for wide deployment in mass-market automobiles, trucks and other vehicles. For example, one commercially available LIDAR sensor is the HDL-64E manufactured by Velodyne (see http://www.velodynelidar.com/hdl-64e.html). The HDL-64E LIDAR sensor is designed for obstacle detection and navigation of autonomous vehicles, such as ground vehicles and marine vessels. It includes sixty-four (64) pairs of lasers and photodiodes to scan and generate a relatively high level of detail of the surrounding environment. The HDL-64E LIDAR sensor is too expensive, however, to be commercially feasible as a sensor for mass market autonomous cars.
Velodyne also manufactures other less expensive LIDAR sensors including the HDL-32E (see http://www.velodynelidar.com/hdl-32e.html). In order to reduce the cost of the sensor Velodyne included thirty-two (32) pairs of laser emitters and photo diode detectors in the HDL-32E instead of sixty-four (64) pairs included in the HDL-64E. With fewer pairs of lasers and photodiodes, the HDL-32E sensor provides a lower resolution view of the surrounding environment than the HDL-64E sensor. Despite including fewer laser/photo diode pairs, however, the Velodyne HDL-32E sensor is still too expensive to be adopted for use in mass-market automobiles.
The fundamental technology behind the HDL-64E LIDAR sensor and the HDL-32E LIDAR sensor is covered by U.S. Pat. No. 7,969,558 (“the '558 patent”) assigned to Velodyne. FIG. 12 of the present application is a reproduction of FIG. 13 of the '558 patent (annotated with new reference numbers), which in turn, provides a general representation of what became the HDL-64E sensor. As shown in FIG. 12, the '558 patent discloses a LIDAR sensor 250 that includes an upper housing 252 supported by a base 258. The upper housing 252 includes sixty-four (64) emitter/detector pairs mounted in two separate assemblies 254 and 256.
As shown in FIG. 13 of the present application, which is cross-sectional view of FIG. 12 and a reproduction of FIG. 14 from the '558 patent, assemblies 254 and 256 are positioned within upper housing 252 at different angles relative to the horizontal to provide different vertical fields of view. Base 258 includes a magnetic motor 259 and a stator 260 and is connected to a rotary coupling 261 that allows motor 259 to rotate the base 258, and with it upper housing 252, enabling assemblies 254 and 256 to capture a full 360 degree horizontal field of view.
Velodyne filed a second patent application on its LIDAR technology that issued as U.S. Pat. No. 8,767,190 (“the '190 patent”). The '190 patent specifically states that it includes several improvements on the technology described in the '558 patent and that the technology disclosed in the '190 patent is incorporated into the HDL-32E LIDAR sensor. FIGS. 14 and 15 of the present application are reproductions of FIGS. 9 and 8, respectively, of the '190 patent (annotated with new reference numbers). As shown in FIGS. 14 and 15, the '190 patent discloses a LIDAR sensor 300 that, like sensor 250 set forth in the '558 patent, includes an upper housing supported by a base 380 along with a rotary component. The upper housing includes thirty-two (32) separate emitter boards 330 and thirty-two separate detector boards 332 mounted to a vertically oriented motherboard 320. The upper housing also includes first and second mirrors 340, 342 that, along with the motherboard 320, are mounted to a common frame 322, and lenses 350 and 352 supported by a lens frame 354.
In operation, the rotary component rotates the upper housing to provide a 360 degree field of view while each emitter fires rearward into first mirror 340. Light reflects off mirror 340 through a hole 324 in motherboard 320 and then through lens 350 before the emitted light 360 travels out to a target 370. After being reflected off target 370, the returned light 362 passes through the detector lens 352 and through motherboard hole 324. The returned light then reflects off the second mirror 342 into the corresponding detector.
The technology described in the '558 and '190 patents (the “Velodyne patents”) and incorporated into the HDL-64E and HDL-32E sensors manufactured by Velodyne has a number of important limitations that limit its ability to be incorporated into a LIDAR sensor that has sufficient resolution and range to be useable in autonomous vehicle applications at a price point and reliability that will enable the technology to be widely adopted in mass-market automobiles. From an overall design standpoint, the architecture described in the Velodyne patents has a low degree of system integration and includes a rotary actuator that is a separate module not integrated into the LIDAR sensor. The lack of integration and the inclusion of a separate rotary actuator requires specialized mounts and interconnects that increase the complexity of the sensor and hence increase the cost.
Another problem with the Velodyne LIDAR sensors mentioned above and described in the above-referenced patents is that each of the emitter/detector pairs in the LIDAR sensors includes a laser diode emitter and an avalanche photo diode (APD) detector. APDs are analog devices that output an analog signal, e.g., a current that is proportional to the light intensity incident on the detector. APDs have high dynamic range as a result but need to be backed by several additional analog circuits, such as a transconductance or transimpedance amplifier, a variable gain or differential amplifier, a high-speed A/D converter, one or more digital signal processors (DSPs) and the like. Traditional APDs also require high reverse bias voltages not possible with standard CMOS processes that typically must be generated by a separate discrete high voltage power supply. Without analogous mature CMOS technology available, it is difficult to integrate all this analog circuitry onto a single chip with a compact form factor. Instead, Velodyne uses multiple external circuit modules located on separate printed circuit boards, which contributes to the high cost of these existing units.
Additionally, due to the required number of physically and electrically separate components associated with each APD detector, Velodyne mounts the separate laser emitter/detector pairs to individual, separate circuit boards. As an example, the Velodyne HDL-64E includes sixty-four emitter/detector pairs and thus includes sixty-four separate emitter boards and sixty-four separate receiver boards. Each such emitter board and receiver board is separately mounted to a motherboard with each emitter/detector pair precisely aligned along a particular direction to ensure that the field of view of every detector overlaps with the field of view of the detector's respective emitter. As a result of this architecture, precision alignment techniques may be required during assembly to align each emitter board and each receiver board separately. The individual, separate circuit boards associated with the APD detectors and laser diode emitters also limit the extent to which the Velodyne LIDAR sensors can be made compact.
This architecture becomes increasingly problematic when one desires to scale the resolution of the device because increasing the resolution requires the addition of more laser emitter/detector pairs, with each mounted on their own circuit board. Consequently, scaling the resolution linearly with this type of architecture can lead to exponential increases in manufacturing costs and also exponential reductions in reliability due to the sheer number of individual parts and boards involved. For example, even if the yield of each individual APD is 99.5% reliability per emitter/APD pair, a device with 32 pairs will be 85% reliable, a device with 64 pairs will be 72.5% reliable and a device with 128 pairs will be 52.6% reliable. Then, once assembly and alignment is complete, great care must be taken to ensure that the precisely aligned multi-board arrangement is not disturbed or jolted out of alignment during shipping or at some other point over the design life of the system. Reliability has been a notable problem for Velodyne sensors.
The Velodyne architecture is also designed so that it preferably employs a time multiplexing scheme that activates only one or a small subset of emitter-detector pairs at any given time. Such an arrangement requires additional timing electronics and multiplexing software and hardware which adds cost and complexity to the system. Time-multiplexing in such a manner can also increase the time between measuring a same 3D position and can potentially fail to adequately identify and warn of fast moving objects.